1. Field of the Invention
This invention relates to an inspection apparatus for detecting foreign matter on a surface to be inspected, and a semiconductor-device manufacturing method using the apparatus. More particularly, the invention is suitable for inspecting the presence and the position of foreign matter, such as opaque dust, or the like, when the foreign matter adheres to an original, such as a reticle, a photomask, or the like, having a circuit pattern formed thereon, which is used in a semiconductor-device manufacturing apparatus, and/or a protective pellicle formed on the original.
2. Description of the Related Art
In an IC (integrated circuit) manufacturing process, IC's are, in general, manufactured by transferring a circuit pattern for exposure formed on an original, such as a reticle, a photomask, or the like, onto the surface of a wafer, on which a resist is coated, by a semiconductor printing apparatus (a stepper or a mask aligner).
At that time, if foreign matter, such as a pattern defect, dust or the like, is present on the surface of the original, the foreign matter is also transferred when the circuit pattern is transferred, thereby causing a decrease in the yield of IC manufacture.
Particularly when a circuit pattern is printed on the surface of a wafer by a step-and-repeat method using a reticle, if even a single harmful foreign matter particle is present on the surface of the reticle, the foreign matter is transferred onto the entire surface of the wafer, thereby causing a great decrease in the yield of the IC manufacturing process.
Accordingly, in the IC manufacturing process, it is indispensable to detect the presence of foreign matter on a substrate, and various kinds of inspection methods have been proposed.
In general, a method of utilizing the property of foreign matter to isotropically scatter light is mostly used.
FIG. 16 is a diagram illustrating the configuration of a principal portion of a conventional foreign matter inspection apparatus for inspecting the presence of foreign matter by detecting light scattered by the foreign matter.
In FIG. 16, a laser beam emitted from a laser light source 151 is converted into a laser beam most suitable for foreign-matter inspection by a polarizer 152, a filter 153, a collimating system 154, and the like, and is guided to a scanning optical system, comprising a scanning mirror 157, such as a polygonal mirror or the like, and an f.theta. lens 158, via a mirror 155. The scanning laser beam from the f.theta. lens 158 is condensed onto the surface of an original to be inspected 160, such as a reticle or the like, having a circuit pattern formed thereon as a scanning spot 159. By relatively moving the original 160 in a direction orthogonal to the scanning direction of the scanning spot 159 by a scanning stage system 166, the entire surface of the original 160 is scanned and inspected.
A detection system, comprising a lens system 161, an aperture 163 and a photoelectric detector 164, is disposed in a backward or lateral direction with respect to the incident direction of the laser beam. The detection system is disposed in a direction such that scattered light, generated from the circuit pattern, or the like when the laser beam is projected onto the original 160, and having particular diffraction directions, is not detected.
FIGS. 17 and 18 are diagrams schematically illustrating the positional relationship between foreign matter on the surface of the original 160, the circuit pattern and the illuminating light beam, and a signal obtained at that time, respectively. FIG. 17 shows the illuminating position on the original 160 from the illuminating side and the light-receiving side, and illustrates a state in which foreign matter 170 and a circuit pattern 171 are present on a scanning line SX of a light beam 20.
In the apparatus having the above-described configuration, when foreign matter is absent within the scanning spot 159, no scattered light is detected by the photoelectric detector 164. When foreign matter is present, scattered light is isotropically generated from the very small foreign matter, and is detected by the photoelectric detector 164. By processing a detection signal obtained at that time by a signal processing system 165, the presence of the foreign matter is inspected.
More specifically, FIG. 18 illustrates the relationship between a signal output I from the photoelectric detector 164 and the illuminating position of the light beam 20. The output I from the photoelectric detector 164 indicates a start of scanning from a starting point X0, and scattered light from the foreign matter 170 at a position X1, where the output I becomes a pulse signal exceeding a predetermined level (slice level) SL.
While the light beam 20 performs scanning from a position X2 to a position X3, the intensity of the output I does not exceed the predetermined level.
By counting pulse signals exceeding the predetermined slice level SL, the amount of foreign matter is detected. By detecting the intensity of the signal, the size of the foreign matter is determined.
In the foreign-matter inspection apparatus shown in FIG. 16, scattered light is, in some cases, generated from the circuit pattern in the direction of the detector. In the conventional method in which only information relating to the intensity of scattered light at that time is simply utilized, the intensity of scattered light from the foreign matter is assumed to be greater than that from the circuit pattern.
When the foreign matter is small, the intensity of scattered light from the circuit pattern becomes greater than that from the foreign matter. Hence, it becomes difficult to discriminate a photoelectric signal representing scattered light from the foreign matter from a photoelectric signal representing scattered light from the circuit pattern by comparing each signal with the slice level.